#include <iostream>
#include <vector>
using namespace std;
class Solution
{
public:
    int numTrees(int n)
    {
        if (n < 3)
            return n;
        vector<int> dp(n + 1);
        dp[0] = 1;
        dp[1] = 1;
        dp[2] = 2;
        for (int i = 3; i <= n; i++)
        {
            for (int j = 1; j <= i; j++)
            {
                dp[i] += dp[j - 1] * dp[i - j];
            }
        }
        return dp[n];
    }
};